1. Field of the Invention
The present invention relates generally to wireless communication systems, and more particularly, to a method and apparatus for efficiently controlling uplink transmission power of a mobile terminal using aggregated multiple uplink carriers.
2. Description of the Related Art
The development of mobile communication systems has enabled users on the move to be accommodated with communication services. Further, mobile communication systems have been developed to support data communication services in addition to standard voice communication services.
Long Term Evolution (LTE) is a broadband packet-based communication technology, which is expected to soon be commercially available, and which is currently undergoing standardization so that it may be utilized in the 3rd Generation Partnership Project (3 GPP).
Referring initially to FIG. 1, a schematic diagram illustrates an LTE mobile communication system. An Evolved Radio Access Network (E-RAN) 110, 112, is simplified with a two-node architecture composed of Evolved Node B's (ENBs or Node B's) 120, 122, 124, 126, 128, and Access Gateways, or Enhanced Gateway General Packet Radio Service (GPRS) Support Nodes (EGGSN) 130, 132. A User Equipment (UE) 101 accesses an Internet Protocol (IP) network 114 via the E-RAN 110, 112.
The ENBs 120-128 correspond to the conventional Node B's of a Universal Mobile Telecommunications System (UMTS). Each of the ENBs 120-128 connects to the UE 101 and performs the functions of a conventional Node B. In the LTE system, all user traffic, including real time services such as Voice over IP (VoIP), is served through a shared channel.
In a data communication service, a resource is allocated in consideration of an amount of data to be transmitted and channel status, which differs significantly from voice communication services. Accordingly, a mobile communication system is provided with a scheduler, which manages resource allocation in consideration of the available resources, channel status, transmission data amount, and the like. Such resource scheduling is required in LTE. A scheduler that is incorporated into a base station is responsible for management of radio transmission resources. ENBs 120-128 of FIG. 1 collect status information of UEs and schedule the UEs based on the collected status information. Each ENB 120-128 controls a plurality of cells.
LTE promises download speeds that are up to 100 Mbps. In order to achieve these high data rates, various studies have been conducted in several areas, including minimization of the number of nodes involved in the connections, simplification of network topology, and close placement of radio protocol to radio channels. LTE also uses Orthogonal Frequency Division Multiplexing (OFDM) as a radio access technology in order to achieve these transmission rates. LTE also utilizes Adaptive Modulation & Coding (AMC) in determining the modulation scheme and channel coding rate in accordance with the channel status of the UE.
Recently, LTE-Advanced (LTE-A) has been introduced to further improve the transmission speed through the use of diverse new technologies in the LTE communication system, such as carrier aggregation. Carrier aggregation is a technique in which multiple downlink and uplink carriers are aggregated for a mobile terminal.
FIG. 2 is a diagram illustrating carrier aggregation in an LTE mobile communication system. Typically, a base station transmits and receives a plurality of carriers within different frequency bands. For example, when an ENB 205 uses a first carrier 215 of which the center frequency is f1 and a second carrier 210 of which center frequency is f2, a UE 220 transmits and receives a signal and transmits data through one of the two carriers in the conventional communication system. When the UE has carrier aggregation capability, it can be allocated more carriers to increase its transmission speed.
Due to the fact that uplink transmission causes inter-cell interference, it is necessary for uplink transmission power to be maintained at an appropriate level. In order to control the uplink transmission power, the base station sends the UE an uplink transmission power control command through the downlink carrier. The uplink transmission power control command carries information for adjusting the transmission power of the UE and may be carried in the form of a separate control signal or by way of a transmission resource allocation signal.
In a conventional mobile communication system, the UE is allocated a single downlink carrier and a single uplink carrier. Therefore, the uplink carrier to which the uplink transmission power control command refers is clear. However, in the advanced mobile communication system described above in which a mobile terminal transmits through multiple uplink carriers, the uplink carrier to which the uplink transmission power control command refers is not clear.